Wave filter



C. H. FETTER Nov. 12, 1929.

WAVE FILTER Filed 06'. 5, 1923 IN VEN TOR A TTORNE Patented Nov. 112. 1 929 UmTsnQsrATEs manna a. sauna, or inrdrnumw, New omens, Assrenon. me new. mama- 2 mom: nun 'rnnnennrn comm, A seasons-r101: or new Yonx wave rmrna Application filed October 5,

A principal object of m invention is 'Ei-ovide a new an 'improve adjustable wave' tor for alternating electric currents of-a variety of frequencies. Another object of my 5' invention is to provide a wave filter that be. adjusted to pass any oneof several ferent frequency ranges as desiredigAnother .1. object of my invention is to providea wave filter. in which by a simple adjus ment it can, be made to pass onlya low. frequency range or an intermediate frequency band or a high frequency ra e. .[Still mother ob ject ofmy invention 1s to provide a wave filter adapted for use in. connection with 5 telephone subscribers lines by which the can receive as they please anyone of severa -dif-' ferent messizgfis put on their .-lines simultaneously. 7 these and various other objects of my invention will be apparent on consideration of a limited number of specific embodiments thereof which I have chosen to disclose in the following specification by way of example. It will be understood that the specification relates to these examples and thatthe invention will be defined in the ap pended claims.

Referring to the drawin Figure 1 is a diagram showing one em odiment of my invention. Fig; 2 is a diagram showing a '30 modification, and Fig. 3 is a diagram showing attenuation as a function of frequency for the a paratus of Fig. 1. To disc use an example of utility of miinvention as well as to disclose an examp of the invention itself, I have shown in Fig. 1 a telephone system adapted for ordinary subscribers service-by connection of individual lines at a central station, on which is superposed a wire broadcasting system to which the subscriber may listen in by making simple adjustments at his subscribers station. The various subscribers lines 11, 12 and 13 extend fromthe respective switchboard jacks 14,15 and 1c. The numeral-20 indicates carrier current generating and transmitting apparatus'which sends out music or monologue or other 'suit-- able broadcast sounds in the form of modu-' lated carrier current comprised within a vwith theappropriate detail.

192a. stun, no, 686,819

and-thence in-multiple' tothe various lines 11, 12 and 13, through the-respective pairs ofcondensers 8,9fa'nd-10; At 23 there is another. carrier currentbroadcast generating and transmitting uipment 1 involving a range of frequencies 24 is interposedas shown.

uipment for the ran on the main conductors 22 and then in multiple on the lines. 11, 12 and 13.

second. "The output of filter 26 oes to the adjustable filter indicated genera y by the ",5

numeral which will now be described in The adjustablefilter here employed is of the so-cal ed ladder type; comprising like series elements in succession and alternately disposed like shunt elements. The proper terminations atthe ends of such a ladder'type filter to minimize reflection effects may be understood to be interposed where the dotted lines are shownat 28 and 29. The switches.

30, 30 and 31 all move together and have three positions of adjustment 1, 2 and 3, under the control of the subscriber by means of a handle 36. Assume first that the adjustment is at position -1 as shown in Fig. 1. In this position it will be seen that the series elements consist of the inductance coils 32, the series condensers 33 being shunted out by. the

switches 31. Also, the shunt elements each consist of the condensers 34, the inductance coils 35' being disconnected at this position of the switches 30.- 'With the series coils 32 andthe shunt condensers 34 efiectively connected as shown the system 27 constitutes-a low-pass wave, ter. Itscut-ofi frequency 0m 22 to 27 kilocycles per second and an 'ap ropriate filter t 18 is a third to kilocycles terat -19. Themodu lated carrier currents within 'these ranges 22-to 27 and 60 to 65 kilocycles are also put go through the high pass filter 50 range of 5 to 10 kilocycles per second. An being 10 kilocycles per second, the broadcast mo I receiving apparatus .25 will receive only the modulated carrier currents sent from the apparatus within the range 5 to 10 kilocycles per second. By pushing the handle 36 to its second or intermediate position, the switches and 30' will make contact with both the coil and the condenser 34, and each switch 31 will break connection with both of-the two opposed contacts with which itis adapted to co-act. This will put the coil; 32 and condenser 33 in seriesin each filter section and thecoil 35 and condenser 34 in shunt in each,

filter section, and the filter 27 will become a 15, band-pass'filter, subjectto the condition that L O =L,U As will be explained presently,

-- the constants for the coils and condensers have been chosen in this specific example to makethe range 20 $30 kilocycles, which oomprehends the range of apparatus 23 and filter 24, but not any part of the ranges of apparatus 20 and filter 21 and apparatus 18 and filter 19. At this adjustment only the output from apparatus 23 will be received at-the subscribers apparatus 25. I

Let the subscriber shift the handle 36. to its third position and the switches 30' will disconnect the condensers 34, the switches 30 will connect with coils 35. and the switches 31 will make connection to shunt out the coils 32. In this condition the only' effective series elements will be the condensers 33 and the only efl'ective shunt elements willbe the coils 35, and the filter 27 will be a high-pass filter with its critical frequency at kilocycles per second. The subscribers apparatus 25 will in this case receive only the output from. the apparatus 18, and both the output from the apparatus 20 and apparatus a 23 will be shunted out by the wave filter 27.

. The formulas for cut-0E frequencies of the different kinds of filters are as follows:

For the low-pass wave filter (corresponding to 27 with switch 36 at position 1),

In F? :1

where L is the series inductance and G is the shunt capacity.

he values for the two cut-ofi frequencies f, and f, of the band filter (obtained with switch 36 at position 2) are determined by the formulas: I V

na-L.0.=;;.%.; (2

If 41.). a

where L L ,,C and C are identified on Fig.

1 and la is a constant.

The formula for the critical or cut-01f frequeue for the hi h-pass filter 36 at its third position) is (with switch wave filter at the outset for a ran e from 20 to 30 kilocycles per second, it fol ows from Equation (5) that the low-pass wave filter From these formulas it can readily be deduced that J Y fll f2 fl H and that I f1f2 f fa f2' f1 Accordingly, by designing the band-pass will cut oil at 10 kilocycles per second and K as 11 are prevented from getting across to I I another line such as 12, by the condensers 8 and 9 which have hi h impedance for low frequency. All voice kept out of the subscribers broadcast receiving apparatus by the high-pass filter 26. High frequency currents go to the subscribers ordinary telephone set S, but they produce no noticeable efl'ect there.

- Referring to Fig. 3, the dotted line gives the attenuation-frequency characteristic of the low-passfilter 26. However, the adjustment isniade at 36, only frequencies above 5 kilocycles per second are passed. At position 1, the attenuation characteristic designated 1) is in effect, at position 2, the charand at position 3, the characteristic designated (3) is ineflect.

'- At zero frequency the characteristic impedance of the low-pass filter of Fig. 1, at

adjustment 1, is

E W/O. (7) At the geometrical mean frequency of its range, that is at frequency 1/ I]; the characteristic impedance of the band filter of Fig. 1 at adjustment 2 is q E... zB- 01 (8) I And at infinite frequency the characteristic atadjustment 3 is Ze /ti, (9) Thus it will be seen that for the three filters requency currents are 1 acteristic designated,(2) and (2') is in effect impedance of the high-pass filter of Fig.1

1o ap arent from the dia at the respective adjustments, the characductances and. capacities in series as shunt 1 elements. At posltion 2 there is a band-pass filter ofthe same type as in Fig. 1, and at position 3 there is a high-pass filter of the type having a condenser for its series element and'a condenser and coil in series for its shunt element. In this case, if f, and f, are the 29 cut-ofi frequencies for the band-pass filter then for the low-pass filter v jf nm Thus if in Fig. 3, f,=10,000 and f -=15,000, then f =4=62O and f =32,400. The filter of Fig. 3 has the advantage over that of Flg.

'1 in that it gives a sharper cut-ofi at the low-- pass and high-pass ad ustments. What is claimed is: 1. A wave filter having successive recurrent sections with at least one-coil and at least one condenser per section and means to change the connections of at least one kind of 7 said reactance elements to adjust the free transmitting range of the filter to a desired frequency range, certain of said coils and conden'sersbeing operatively connected in on each of several difierentadjustments.

2. A wave filter having successive recur rent sections with at least one reactance element per section in series and at least one reactance element per section in shunt and means to change the connections of the series reactances and the shunt reactances to adjust the free transmitting range of the filter to a desired frequency range, certain of said reactances being operatively connected in at several different-adjustments.

3. A wave filter having successive recurrent sections with at least one coil and at least one condenser per section, and means to change the connections of at least one kind of said reactance elements to adjust the free transmitting range of the filter to 'a desired frequency range, the elements of the combination being determined to make the characteristic impedance substantially thesaine at each adjustment within the respective corresponding free transmitting range, certain of said.

coils and condensers being 0 eratively connected m on each of several d fferent adjustments.

4. A' wave filter having. successive recurrent sections with at least one coil and at least one condenser per section, and means to change the connections of at least one kind of said reactance elements to adjust the .free transmitting range of the filter to a desired frequency range, the'ran e at each adjustment being exclusive of the ranges at other adjustments, certain of said coils and condensers being operatively: connected in. on each of several dlflz'erent adjustments.

5. A wave filter having successive recurrent sections with a coil L and a condenser C, adapted to be connected in series *per section and a coil L anda condenser G, adapted to beconnected in shunt per section subject to the condition, L 0,==L U i] whereby when so connected the filter. is a and-passifilter,

' and means to change the connections of said.

coils and condensers to'convert the filter into a low-pass filter or a high-pass filter.

6. A wave filter having successive recur rent sections with a coilL and a condenser C adapted to be connected in series per section and a coil L and a condenser 0, adapted to be connected in shunt per section subject to the condition L, 6",=L 0 whereby when so connected the filter is a band-pass filter,

and means to change the connections of said.

coils and condensers to convert the filter .into a low-pass filter of range lying considerably below the range of the band-pass filter orto convert itintoahigh-pass filter of. range lying considerably above the range of the bandpass filter.

7. A wave filter having successive recur rent sections with a plurality of reactance elements per section, and means to connect said elements in three difierent ways so that one way the filter will be a low-pass filter,

another way it will be a band-pass filter of higher range and the third way it will be a high-pass filter with its cut-off frequency higher than the band-pass range, certain of said reactances being operatively connected in at several different adjustments.

8. A wave filter having successive recurrent sections with a plurality of reactance elements per section and means to connect said elements in three different ways so that one way the filter will be a low-pass filter, another way it will be a band-pass filter of higher range and the third way it will be a high-pass filter with its cut-off frequency higher than the band-pass range, the values of said reactances being chosen so that at representative frequencies within each of the said ranges the characteristic impedence will have'substantially the same pure resistance Y 9. A wave value, certain. of said reactances being operatively connected in at several different adjustments. e

filter having successive recurrent sections with at least one coil and at least one condenser per section, switches to change the connections of said coils and condensers within each section to adjust the'free transmitting range of the filter to a desired frequency range and a handle to operate said switches simultaneously and in cocrdination, certain of said coils and condensers bein operatively connected in on each of severa dlfl'erent adjustments. 10. A wave filter having successive recurrent sections with a coil and a condenser per section available forseries connection and with a coil and a condenser per section available for shunt connectlon, and means to connect said coils and condensers to make the filter have difierent free transmittingranges of frequency, certain of said coils and condensers' bemg operatively connected for different ranges.

11. A wave filter having successive recurrent sections with a coil and a condenser per section available for series connection, and with a coil and a condenser per section available for shunt connection, and means to con-' nect sald 0011s and condensers to make a low pass filter or a band pass filter or a high pass filter, with the free transmitting range of the band ass filter between the critical frequencies 0 the low pass and the high pass filters.

In testimony whereof, I have signed my name to this specification this 3rd day of October, 1923.

CHARLES H. FETTER. 

